Portland masonry cements are compositions produced particularly for use by masons in the bonding of bricks, blocks, and other masonry units. Such cements are typically mixed prior to use with a fine aggregate, usually sand, and water. It is desirable that the wet mortar have a high degree of plasticity for working (e.g., trowelling) by the mason, as well as other desirable properties such as good "board life" (i.e., a long working time).
Masonry cements are produced by intergrinding Portland cement (containing gypsum for proper setting time regulation), with, generally, from about 20 to 65%, based on the total weight, of a solid material such as limestone, chalk, talc, pozzolans, clay, gypsum, or combinations of such. Limestone is most often the ingredient interground with the Portland cement because of its good plasticity enhancing properties. Such masonry cements are ground to a greater degree of fineness than most Portland cements intended for use in structural concretes. The finer grind of masonry cements improves the plasticity of the finished mortar products.
As discussed in U.S. Pat. No. 5,294,256, additive products are sold for mixture with the masonry cement to add certain desirable properties to the masonry cement composition. It is common practice to mix such additive products with the cement during the intergrinding of the cement with the gypsum, limestone, etc.
In U.S. Pat. Nos. 3,865,601 and 3,885,985, additive products of the above type are described. The additives described therein comprise an aqueous oil-in-water emulsion containing water, a water-insoluble, water-repelling acid component (e.g., tall oil), an emulsifier (e.g., a salt of such an acid), and a setting time retarding agent (e.g., sucrose). This additive product is advantageously dispersible in water which reduces the risk of it being overdosed. The additive may also contain an air-entraining agent as an additional optional component.
An improved emulsion additive product is described in U.S. Pat. No. 4,375,987. In addition to the above constituents, this additive comprised an emulsion stabilizer such as glycol which prevents an oil-in-water emulsion from destabilizing when exposed to freezing temperatures during shipping or storage prior to use.
As explained in U.S. Pat. No. 5,294,256, it has been necessary to add further water retention agents, such as cellulose ether, during the grinding process, to improve the plasticity of Portland masonry cements. Conventional water retention agents cause the emulsion additives described above to destabilize and hence must be added as a separate feed stream, often as a dry powder. This method is inconvenient, may increase manufacturing costs, and increases the risk of overdosing the batch with the water retention agent. These conventional water retention agents also may entrain large quantities of air, which may be undesirable where high strength is concerned. Thus, U.S. Pat. No. 5,294,256 taught the use of an improved additive comprising an oil-in-water emulsion having a solids portion comprised of a major proportion of a water-insoluble, water-repelling acid component selected from the group consisting of fatty acids, rosin acids, and mixtures thereof, a set retarding component for the hydraulic cement, and a polymer selected from polyvinyl acetate, polyvinyl alcohol, hydroxypropyl substituted polysaccharides, or mixtures thereof.
Despite the availability of various mortar cement admixtures, the present inventors believe that a novel additive is needed for obtaining a mortar cement that has a satisfactory workability from the subjective viewpoint of the mason who is applying the mortar between the bricks or other masonry units, while providing for a relatively low air content (when compared to masonry cements) to maintain mortar strength. In particular, it is desired to discover a mortar cement additive which can obtain a maximum air content of 14% or less, as required in Type S and Type M mortar cements, as summarized in the table below:
Type N Type S Type M Masonry cement 21 19 19 Mortar cement 16 14 14
Cement manufacturers need to be especially careful when targeting the 14% maximum air content mark, and typically seek a maximum air content of about 12% as safety measure to avoid exceeding the 14% threshold whereby a weakened masonry structure is obtained Accordingly, the need for a novel mortar cement additive is heightened somewhat in view of the need for obtaining a desired strength as provided by a relatively low air content while retaining workability in the mortar.